Calculating the spacing between stair spindles is one of the easier tasks when installing a new stair railing. The first step is deciding how many spindles you want or need for a particular run. This may be determined aesthetically or by government regulations that set a maximum distance between spindles to keep children from falling through. Once the number of spindles is determined, the spacing is determined by relatively simple arithmetic.

The UK building regulations specify a maximum gap between spindles of 99 mm (3.9 inches).

Measure the horizontal distance between the base of two consecutive support posts -- that is, the two end posts that border the area where the spindles will be installed.

Measure the width of your spindles.

Add the width of your spindles to the spacing width you prefer. The spacing width might be the regulation maximum or an amount chosen for aesthetics. Divide this into the distance between posts to calculate the number of spindles required. As an example, if you had a distance of 300 cm (120) inches with spacing of 10 cm (4 inches) and spindle width of 5 cm (2 inches), you would divide 300 by 15 to calculate 20 spindles.

Multiply the spindle width by the number of spindles. Subtract this figure from the distance between posts. This will give you the cumulative amount of spacing. In the example, you would multiply 5 cm (2 inches) per spindle times 20 spindles to calculate 100 cm (40 inches). You would then subtract that from 300 to give you 200 cm (80 inches) of total spacing.

Add one to the number of spindles to calculate the number of spaces required. Divide this figure into the total amount of spacing to calculate horizontal spacing between spindles or posts. In the example, 200 cm (80 inches) of spacing divided by 21 gives you a horizontal spacing of 9.5 cm (3.81 inches).